1. Field of the Invention
The present invention relates to the field of protection circuits that protect against electric surges that may affect components and/or electronic devices such as, for example, mobile phones, computers, printers. It is particularly desired to protect such devices against electrostatic surges which can, for example, correspond to current peaks, that have short durations but very high intensities, for example, several tens of amperes.
2. Discussion of the Related Art
Conventionally, as is shown in FIG. 1A, to protect inputs 1 and 2 of a device 3 connected to external components, an avalanche diode 5 or an equivalent device is disposed between these inputs. Resistors 7 and 8 correspond to connection resistances or to resistors voluntarily inserted in the circuit.
Theoretically, as is shown in FIG. 1B, an avalanche diode requires a voltage V.sub.Z between its terminals to be conductive. However, when the current through this diode significantly increases, as is shown in FIG. 1B, voltage V across the avalanche diode increases to reach a value V.sub.S equal to V.sub.Z =R.sub.Z I, where I is the current associated with the surge. To take a practical example, considering an avalanche diode of a surface area of approximately 0.4 mm.sup.2, in conventional technologies, the equivalent dynamic resistance R.sub.Z will be on the order of 0.4 ohm. If the current peak is on the order of 30 amperes, the effective voltage across an avalanche diode having a breakdown voltage of 6 volts will be 6+0.4.times.30=18 volts. The effective value is triple the nominal voltage and a destruction of the components of the circuit to be protected can result therefrom.
It is known that to reduce the dynamic resistance of an avalanche diode, its surface should be increased. If the surface is increased by a factor 4, to reach for example a surface of 1.6 mm.sup.2, the equivalent resistance will be of 0.1 ohms only. The overvoltage linked to the current flow will be of 3 volts only, that is, the voltage across the avalanche diode will reach, for 30 amperes, a value on the order of 9 volts, which remains acceptable.
However, this has the disadvantage that the cost of a diode increases with its surface and that a significant surface has to be provided so that the increase of the voltage with the current becomes negligible.
Further, an avalanche diode inevitably exhibits in the blocked (non-conducting) state a certain stray capacitance. In a conventional case, for a diode of 0.4 mm.sup.2, there will be a stray capacitance on the order of 250 pF. If the diode surface is multiplied by 4, the stray capacitance will be multiplied by four to reach a value on the order of one nanofarad. As a result, the protection system exhibits a high capacitance which cannot be chosen independently from the choice of the resistance. This can be a disadvantage in some applications.